Essential elements of a generic ground milling machine include a machine frame, traveling devices connected to the machine frame, a drive engine, a ground milling device with a milling drum box open towards the ground and a milling drum that is mounted within the milling drum box and that is rotatable about a rotational axis extending horizontally and transversely to the working direction, the ground milling device being adjustable via an adjusting device between a lifted transport position and a milling position engaging the ground. The traveling devices are usually wheels and/or crawler tracks. They are usually arranged in front and rear pairs, although there can also be only a single front or rear running gear. It is further known to configure at least one of the running gears in such a way that it is adjustable relative to a side wall of the machine between an inner end position and an outer end position. The drive engine, typically a diesel engine, provides the drive power required for the operation of the self-propelled ground milling machine. The actual milling operations are performed with the aid of the rotating milling drum, which is, e.g., a supporting cylinder equipped with a plurality of milling tools on its outside jacket surface. The ground milling device is height-adjustable relative to the ground. To this end, the ground milling device can be adjustable relative to the machine frame and/or the traveling devices on the machine frame can be adjustable, in particular by means of lifting columns, so that the entire machine frame can be lifted and lowered. In the milling position, the ground milling device is lowered far enough toward the ground for the milling drum to engage the ground at a desired milling depth. In the transport position, in contrast, the milling drum is lifted out of the ground and kept at a relatively large vertical distance from the ground so that it is possible, e.g., to drive over small obstacles extending in the vertical direction, such as e.g. milling edges etc., while avoiding an undesired collision of the milling drum with the ground. The ground milling machine further comprises a conveyor device with at least one primary conveyor belt, with which milled material produced in the milling drum box can be transported away from the ground milling device, the primary conveyor belt including a receiving side and a discharge side. The receiving side refers to the end of the longitudinal primary conveyor belt at which milled material is loaded, in particular directly from the milling drum box, onto the primary conveyor belt. Accordingly, the discharge side refers to the end of the primary conveyor belt at which the milled material is discharged from the primary conveyor belt. The discharge can occur onto an additional, subsequent conveyor belt, for example an attachable conveyor belt, or onto a transport vehicle, or also onto the ground. Also provided is a belt holder, which is arranged in the working direction of the ground milling machine in front of the ground milling device and in which the primary conveyor belt is mounted on the receiving end side. In particular in the case of so-called compact milling machines, e.g. the BM500 and BM600 models of the applicant, the belt holder can be positioned behind the milling drum since the loading of, e.g., the truck occurs to the rear and, accordingly, the belt is located behind the milling machine and is oriented to the rear. The belt holder can form part of a so-called retaining device, which is usually a grate-like member skimming over the ground directly in front of the milling drum box during milling operation in order to prevent blocks of ground material from breaking away directly in front of the ground milling device. It is also possible, and comprised by the invention, to configure the belt holder in such a manner that it is separate from the retaining device, e.g., by means of a link connected to the milling drum box and/or the machine frame. Finally, the generic ground milling device comprises an operator platform, from which an operator can operate the ground milling machine in working operation. Generic ground milling machines of this type are disclosed, e.g., in DE102015007937A1, DE102015007562A1 and DE102014019168A1.
A precise steering of the ground milling machine during milling operation, e.g. in order to facilitate a precise milling along markings and/or obstacles such as curbs etc., is essential when using such ground milling machines. In this regard, it is known to use reference markings attached to the sides of the machine that are visible to the driver from the operator platform and that provide him/her with, e.g., a point of reference as to where a face side of the milling drum housed inside the milling drum box extends. Problematic, however, are in particular obstacles that lie within the milling width, e.g. manhole covers, etc. These obstacles are thus essentially merely “occasional” and are located within the area of the ground region to be milled, and thus need to be driven over by the ground milling machine during the milling process. It is not possible for the operator of the ground milling machine to mill along or past these obstacles, which will also be referred to as milling area obstacles in the following. Rather, in order to prevent damage to the ground milling device and/or the milling area obstacle itself, it is necessary to lift the ground milling device out of the milling position and into the transport position before the area obstacle in question, when viewed in the working direction, and, afterwards, to lower the ground milling device back into the milling position. In particular in order to minimize time-consuming subsequent work, it is advantageous here to mill in the working direction as closely to the area obstacle in question as possible and to resume the milling operation as closely as possible behind the area obstacle. However, as the milling drum is arranged inside the milling drum box, it is not possible for the driver of the ground milling machine to see the exact position of the milling drum and in particular its cutting edges transversely to the milling direction. Instead, he needs to estimate the approximate distance between the milling area obstacle and the front and/or rear cutting edge (i.e. the cutting circle) of the milling drum in the working direction of the ground milling machine. The concomitant inaccuracies increase either the subsequent manual work required or the risk of damage to the milling drum and/or the respective area obstacle.